Copper-containing sorbents play a major role in the removal of contaminants, such as sulfur compounds and metal hydrides, from gas and liquid streams, such as synthesis gas. The active copper phase for the removal of sulfur compounds from synthesis gas can be derived from copper compounds, mainly in carbonate, oxide and hydroxide form or mixtures thereof. Copper adsorbents for synthesis gas are usually porous solids with well-developed pore structure and appreciable surface area. Inorganic supports or binders can be used to provide physical stability and durability at the process conditions of synthesis gas purification.
The term synthesis gas designates mixtures of carbon monoxide (CO) and hydrogen (H2) in varying proportion which often contain carbon dioxide (CO2), and water (H2O). The most typical process of synthesis gas production consists of high temperature reforming of natural gas or other hydrocarbon feeds. The synthesis gas is then fed to different catalytic processes such as low and high temperature water shift reactions which are susceptible to catalytic poisons, mainly hydrogen sulfide (H2S) and carbonyl sulfide (COS).
Synthesis gas may be produced from various processes. For example, copper containing catalysts are widely used to catalyze the low temperature water shift reaction in which carbon monoxide is reacted in presence of steam to make carbon dioxide and hydrogen. Copper catalysts are also used in the synthesis of methanol and higher alcohols. Producing synthesis gas from coal is another commercial technology. In such processes the product stream contains a range of contaminants, with arsine (AsH3) being the most detrimental for the catalytic process downstream. A typical raw synthesis gas stream contains approximately 0.5 to 1.0 ppm arsine. Coal derived synthesis gas may in some instances contain mercury and heavy metals as contaminants.
Synthesis gas requires frequently adding hydrogen sulfide in order to prevent metal dusting corrosion which is known to occur at temperatures over 300° C. (572° F.). However, hydrogen sulfide is poisonous to the downstream catalysts and needs to be removed at a level of approximately 20 ppb or less. Thus, synthesis gas typically contains various contaminants such as hydrogen sulfide, arsine, and mercury.
It is known to use copper containing adsorbents to remove the hydrogen sulfide from synthesis gas. For example, U.S. Pat. No. 7,323,151 discloses such an adsorbent. Unfortunately, the reducing agents contained in the synthesis gas, such as carbon monoxide and hydrogen gas, can trigger the reduction of the oxide to the copper metal which is less suited for removal of other contaminants such as arsine and mercury. A further detriment to the reduction process is that heat is liberated which may result in runaway reactions and other safety concerns in the process.
The presence of cupric oxide (CuO) is important so that, in the presence of hydrogen sulfide, the cupric oxide can form cupric sulfide (CuS). The cupric sulfide is desirable for its ability to remove mercury.
Thus, easily reducible cupric oxide is disadvantageous in the purification of synthesis gas. Again, the removal of some hydrogen sulfide from gas streams at elevated temperatures is based on the reaction of cupric oxide with hydrogen sulfide. Thermodynamic analysis shows that this reaction results in a low equilibrium concentration of hydrogen sulfide in the product gas even at temperatures in excess of 300° C. (572° F.). The residual hydrogen sulfide concentration in the product gas is much higher (which is undesirable) when cupric oxide reduces to copper metal in the course of the process since the reaction, below, is less favored than cupric oxide sulfidation to cupric sulfide:2Cu+H2S=Cu2S+H2.
Combinations of cupric oxide with other metal oxides are known to retard reduction of cupric oxide. However, this is an expensive option that lacks efficiency due to performance loss caused by a decline of the surface area and the lack of availability of the cupric oxide active component. The known approaches to reduce the reducibility of the supported cupric oxide materials are based on combinations with other metal oxides such as chromium (III) oxide (Cr2O3). The disadvantages of the approach of using several metal oxides are that it complicates the manufacturing of the sorbent because of the need of additional components, production steps and high temperature to prepare the mixed oxides phase. As a result, the surface area and dispersion of the active component strongly diminish, which leads to performance loss. Moreover, the admixed oxides are more expensive than the basic cupric oxide component which leads to an increase in the sorbent's overall production cost. Accordingly, in spite of the aforementioned shortcomings associated with using copper metal adsorbents, such as in U.S. Pat. No. 7,323,151, these adsorbents remain one of the most commonly used adsorbents for removing hydrogen sulfide.
Therefore it would be desirable to have an adsorbent that is capable of effectively and efficiently removing multiple contaminants from the synthesis gas. Accordingly, the present invention is intended to solve one or more of these problems.